In general, when collimated light is focused, a spot of light is generated having a generally defined axial and lateral extent that is limited by diffraction. An extended depth of focus beam is, by one definition, one in which the axial extent of the focal spot is increased by an arbitrary factor, which is generally greater than 5 but typically less than 20, without significantly compromising the lateral extent. For example, where with a normal lens an axial extent of about 1.5 microns and a lateral extent of approximately 0.5 microns may be achieved, an extended depth of focus beam may have an axial extent of about 15 microns and a lateral extent of about 0.5 microns.
For purposes of the present invention, while the term “light” refers generally to the visible portion of the electromagnetic spectrum, “light” as used herein also pertains to non-visible portions of the electromagnetic spectrum including, but not limited to, radiation in infrared and ultra-violet portions of the spectrum.
Extended depth of focus light beams have application in scanning optical microscopes. A scanning optical microscope is an instrument in which a focussed spot of light is scanned across an object and the light reflected from that object (or emitted by fluorescence) is collected, for example by means of a photodetector and assembled to form an image of the object. The resolution of the technique is obviously limited by the diameter of the light beam, and the depth of focus of the light beam limits the maximum size of an object that can be imaged without refocusing.
Various methods exist for generating focussed light beams. One of these methods uses an optical device called an axicon. This is a refractive element having a plane face and a conical face. When homogeneous light from a laser is passed through the axicon, it is bent inwards and crosses the optical axis at a point that is proportional to the distance from the optical axis at which the light passed through the axicon. Constructive interference takes place where the light crosses the optical axis, producing a very narrow light beam with an extended depth of focus.
The disadvantage of the axicon device is that the intensity of the light in the beam varies significantly with distance along the optical axis, which limits the usefulness of the device.
Another known method of generating a light beam with an extended depth of focus involves the use of a diffractive optical element having a set of concentric amplitude or phase rings. However, like the axicon, the intensity of the light beam again varies significantly with distance along the optical axis.
Yet another known method of generating a light beam involves the use of an annular aperture that is placed in the back focal plane of the objective lens. This produces an extended focal depth beam with good axial uniformity of intensity. However, the method is highly inefficient, since most of the light is blocked by the annular aperture mask.